For many years, glass melting furnaces generally, and in particular those used for making electric lamp bases, consisted of a hearth for melting and refining the glass and a platinum orifice to permit the flow of glass out of the furnace. The glass flow rate was entirely a function of the diameter and length of the orifice, the glass composition, depth and temperature.
Difficulties with this design have been encountered as far as the inability to rapidly and accurately change the glass flow rate, which could only be changed by changing the temperature of all the glass in the furnace.